1. Field of the Invention
The present invention relates to an outboard motor that propels a vessel.
2. Description of the Related Art
US 2014/0322997 A1 discloses an outboard motor powered by a V-type six-cylinder engine. This outboard motor is equipped with an in-bank exhaust system that discharges exhaust gases to the inside of two cylinder banks.
The exhaust device of the engine includes exhaust passages, the number of which changes in order of 6-2-1-2-1. In detail, three exhaust ports corresponding to the right cylinder bank join together at the right exhaust manifold, and three exhaust ports corresponding to the left cylinder bank join together at the left exhaust manifold. The two passages provided in the two exhaust manifolds join together at the upstream portion of the exhaust pipe, and are branched into two at the downstream portion of the exhaust pipe. The two branch passages branched at the downstream portion of the exhaust pipe pass through two cylinder heads, and join together at the cylinder body. Thereafter, the exhaust passage passes through the exhaust guide and extends from the cylinder body to the propeller.
However, in the outboard motor disclosed in US 2014/0322997 A1, since the exhaust passage is branched and then joins together again, a pressure loss of the exhaust gases, that is, resistance applied to the exhaust gases increases. This causes a decrease in the engine output and a deterioration in fuel consumption.
For example, if one of the two branch passages is closed, branching and joining together again of the exhaust passage is eliminated. However, in this case, since exhaust gases that should be discharged to the two branch passages are discharged to only one branch passage, the pressure loss of the exhaust gases increases due to a decrease in the flow passage area. In addition, since the branch passages are provided in the cylinder head and the cylinder body, the flow passage area of the branch passages cannot be significantly increased while preventing influences on other portions.